This invention relates generally to gas purification, and more particularly to methods and apparatus for removing impurities from noble gasses, nitrogen, and other non-reactive gasses.
In the semiconductor manufacturing industry pure gasses are used in a variety of manufacturing processes, such as chemical vapor deposition (CVD), plasma etch, etc. The purity of the gas used in the manufacturing process becomes more critical as the feature width of integrated circuits decreases. For example, a decade ago feature widths in the range of 3 to 5 microns were standard. Currently, integrated circuits having feature widths of 0.5 to 0.7 microns are in production. With smaller feature widths, even a very low level of contaminants can damage an integrated circuit, thereby destroying its functionality or degrading its performance. Typical contemporary process specifications require process gasses to have less than 10 parts per billion (ppb) of contaminants, and preferably less than 1 ppb of contaminants.
One type of gas purifier utilizes hot getter materials for the removal of impurities from noble gasses and nitrogen. The getter materials are encased in stainless steel containers which are typically heated to a temperature in the range of 300.degree. to 450.degree. C. Unfortunately, stainless steel outgasses a significant amount of hydrogen at temperatures above approximately 200.degree. C. In the past when process specifications allowed 100 ppb of hydrogen in a purified process gas this was not a major problem. However, with contemporary process specifications, the hydrogen outgassed from hot stainless steel surfaces has become a significant problem.
After a process gas passes through a getter-type gas purifier, it is cooled in a heat exchanger. Absent such cooling, the hot gas would pick up more hydrogen from the stainless steel tubing connecting the gas purifier to the semiconductor processing equipment and could also possibly damage valves and gas panels of the equipment. Prior art heat exchange units used in conjunction with semiconductor manufacturing equipment include a single heat exchange coil made from stainless steel tubing. Typically, the coil is immersed in a container through which a coolant liquid, such as water, is pumped.
An advantage of coiled heat exchangers is that they are relatively non-contaminating because the stainless steel coils have relatively few welds (e.g. typically only one at each end) which might leak or generate particles. Predictably, coiled heat exchangers also have some disadvantages. One of the main disadvantages of coiled heat exchangers is that the act of coiling the stainless steel tubing can compromise the electropolish finish on the interior surface of the tube, thereby increasing the amount of particulates produced by the tube itself. Another disadvantage of these prior art heat exchangers is that the long length of tubing can create a substantial back-pressure to the gas purifier unit.
Another type of heat exchanger, which is not known to have been used in the semiconductor processing industry with purified gasses, is the parallel tube heat exchanger. One type of parallel tube heat exchanger utilizes preformed T-connectors which are welded together to form inlet and outlet manifolds. A number of parallel, straight tubes are coupled between the two manifolds. Another type of parallel tube heat exchanger utilizes a pulled-T system. In such a system, holes are punched through from the inner surface of a manifold, and a short connecting nipple is welded to the manifold at each punched hole. Often, a reduction fitting is welded to each nipple to provide the appropriate diameter for the straight tubing. With the above-described types of parallel tube heat exchanger the number of welds are in the range of 4n+2 to 6n+2 to connect n tubes between two manifolds.
The large number of welds required of prior art parallel tube heat exchangers are more likely to be contaminating than the relatively few (typically two) welds required of coiled heat exchangers. Because of the contamination problems with parallel tube heat exchangers of the prior art, they have been thought to be poor choices for use with gas purification systems in the semiconductor manufacturing field.
Nonetheless, parallel tube heat exchangers have certain advantages over coiled tube heat exchangers. For one, parallel tube heat exchangers exhibit much less back-pressure for a given heat removal capacity than coiled tube heat exchangers. The parallel tube heat exchangers also tend to exhibit less of a temperature gradient along their relatively short lengths, thereby reducing the surface area of the tubes exhibiting temperatures greater than about 200.degree. C. where hydrogen outgassing becomes significant.